The present invention relates generally to hybrid direct current (DC) contactors and, more particularly, to a system and method for controlling operation of a hybrid DC contactor that improves performance of the hybrid contactor.
Electro-mechanical contactors are used in a variety of environments for turning on and off a power source to a load electrically. The contactors include movable contacts and fixed contacts. The movable contacts are connected to an electromagnet and are controlled to selectively turn on or off power from the source to the load. The contacts are typically maintained in an open position by way of a spring and are caused to translate to a closed position when power to the electromagnet's coil is applied.
When electro-mechanical contactors are used for interrupting AC currents, it is recognized that there is always a time when the current becomes zero. Electro-mechanical contactors can thus interrupt current at the zero current and when the contacts separated. However, when electro-mechanical contactors are used in a DC voltage system, an electric arc may form in the space between contacts during transition of the movable contacts between the closed and open positions. Without intervention, this arc will continue until the separation between the contacts is too large to sustain the arc. When interrupting DC current, the separation between the fixed and moving contacts has to be large (in air, under standard pressure conditions). Thus, it is known to experts in the field that, for interrupting DC currents, special magnets are required in DC contactors.
To address the issue of not being able to interrupt the current caused by arcing, hybrid DC contactors have been developed that incorporate a solid state device that is connected in parallel with the mechanical main contacts. The solid state device may, for example, include an IGBT switch, a snubber capacitor, and a snubber resistor. In operation, when changing the mechanical main contact from the closed state to the open state, the solid state device that is connected in parallel to the mechanical main contact is turned on first. The current flowing through the mechanical main contact is thus caused to flow through the solid state device. Next, the mechanical main contact is allowed to open by removing the voltage applied to the electromagnet coil that controls positioning of the mechanical main contact. By turning on the solid state device prior to opening the mechanical main contact, the voltage on both ends of the turned-on solid state device and the mechanical main contact can be opened with only a minimal voltage not sufficient to form an arc.
While existing hybrid DC contactors do function to provide a bypass path to the mechanical contacts, there are drawbacks to the design and control of such hybrid DC contactors. One such drawback to existing hybrid DC contactors is that, when bypassing the main contacts, the separation between the moving and fixed contacts cannot be determined. Not knowing the separation, the sold state device is left closed for a fixed but long enough delay to ensure interruption of the current. As such, the full current value needs to flow through the solid state switch for several milliseconds, necessitating that the solid state switch be oversized to handle several milliseconds of current. This oversizing of the IGBT switch increases the production cost of existing hybrid DC contactors.
Another drawback to existing drawback hybrid DC contactors is that the switching time of the contactor is prolonged enough that the contacts may still be exposed to an undesirable “restrike” of arcing. That is, when interrupting DC currents due to the inductance in the circuit, the voltage across the main contacts rapidly rises, and this rapid rise in voltage can cause a breakdown of the air gap between the fixed and moving contacts called “restrike.” The fixed and moving contacts have to be separated by a sufficient gap to prevent such a restrike (that is based on contactor design and other conditions). Still another drawback of the prior art hybrid DC contactors is that, if there is restrike of the arc, there is no capability to know the condition and this could result in the burning out of the contactor. Still another drawback to existing drawback hybrid DC contactors is that they do not provide galvanic isolation, which is desirable in some implementations of hybrid DC contactors. Still another drawback is that the existing design of hybrid DC contactors is not suitable for bidirectional currents.
It would therefore be desirable to provide a hybrid DC contactor system circuit and method for controlling thereof that reduces the time the solid state device carries the current, provides the capability to determine if the gap between the fixed and moving contacts is enough not to cause a restrike, provides for detection of a strike and ensures to turn on the solid state switch once again to ensure that the current is interrupted, provides galvanic isolation, and is suitable for bidirectional currents. Such a circuit could advantageously be applied to a breaker that can trip due to an over current or by a shunt trip. When the arc is detected, the circuit automatically waits, pulses the IGBT, and interrupts the current. The circuit could also be configured as a bidirectional circuit that is applied to circuit breakers as well. This hybrid contactor coil can be opened to activate the circuit with a number of detection circuits such as the following commonly used: over-current detectors, over-voltage or under-voltage detectors, and ground fault detectors.